jit: Drop redundant LLVM configure probes.

jit: Drop redundant LLVM configure probes.

We currently require LLVM 14, so these probes for LLVM 9 functions
always succeeded.  Even when the features aren't enabled in an LLVM
build, dummy functions are defined (a problem for a later commit).

The whole PGAC_CHECK_LLVM_FUNCTIONS macro and Meson equivalent are
removed, because we switched to testing LLVM_VERSION_MAJOR at compile
time in subsequent work and these were the last holdouts.  That suits
the nature of LLVM API evolution better, and also allows for strictly
mechanical pruning in future commits like 820b5af7 and 972c2cd2.  They
advanced the minimum LLVM version but failed to spot these.

Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGJgB6gvrdDohgwLfCwzVQm%3DVMtb9m0vzQn%3DCwWn-kwG9w%40mail.gmail.com